CONSTRAINTS on THE NATURE OF THE SUBVOLCANIC SYSTEM AT SOUTH SISTER VOLCANO, OREGON FROM 238U-230Th ZIRCON AGES AND 238U-230Th-226Ra PLAGIOCLASE AGES

We present in situ ²³⁸U-²³⁰Th zircon ages and bulk ²³⁸U-²³⁰Th-²²⁶Ra plagioclase ages from the Holocene eruptions of the Rock Mesa (RM) and Devil’s Hills (DH) rhyolites on the southern flank of South Sister volcano, Oregon. In situ major and trace-element analysis of zircon and plagioclase provide the context necessary to interpret the ages in terms of the nature of the subvolcanic system and mechanisms of rhyolite genesis at South Sister. The RM and DH flows are small volume rhyolites which erupted at 2300yBP and 2000yBP respectively (Scott, 1987). Zircons from the RM and DH dominantly record ages from 20-80ka and >350ka, whereas plagioclase records ²³⁰Th-²²⁶Ra ages of 3600-6900yBP (RM) and 4000-8900yBP (DH) and a ²³⁸U-²³⁰Th age of 13±15ka (DH). Zircons <350ka are interpreted to be antecrystic grains recycled from older South Sister magmas due to the undersaturated nature of the RM and DH magmas with respect to zircon, the paucity of young zircons, and age ranges which overlap with periods of silicic volcanism at South Sister. Zircons >350ka are interpreted to be xenocrystic since they predate the onset of volcanism at South Sister. In contrast to zircon, plagioclase records young ages which imply some plagioclase crystallized close to the time of eruption; however, scatter in LA-ICPMS trace-element data which is greater than can be explained by partitioning behavior and the slightly older ²³⁸U-²³⁰Th age suggests the presence of antecrystic plagioclase which were recycled from a source of similar composition.

The abundance of antecrystic zircon and plagioclase suggests that a large proportion of the grains in the RM and DH were recycled from previous South Sister magmas. Furthermore, the RM and DH glasses have distinct (²³⁰Th)/(²³²Th) despite similar having similar crystal ages and whole rock chemistries, implying these two eruptions were not generated from a single, closed system magma chamber. These observations favor a model for rhyolite genesis at South Sister dominated by remelting/remobilization of previous magma batches and imply that zircons record information about crystallization in the subvolcanic mush zone as opposed to the host magma, whereas plagioclase records younger ages weighted towards autocrystic growth in the host magma.